Method of preparation of filaments from polytetrafluoroethylene emulsion



United States Patent METHOD OF PREPARATION OF FHLAMENTS FROM POLYTETRAFLUORQETHYLENE EMUL- SIGN Clarence Boyer, Swarthrnore, P2,, assignor to E. I. the Pont de Nemours and Company, Wilmington, Bot, a corporation of Delaware No Drawing. Filed Dec. 24, 1956, Ser. No. 630,082

2 Claims. (Cl. 264-211) The present invention relates tothe formation of films, fibers and the like from tetrafiuoroethylene polymers. Specifically the invention is directed to a process of making continuous structures from an aqueous polytetrafluoroethylene dispersion which contains emulsified in it a water-immiscible organic solvent containing an organic polymer.

Throughout the description of this invention, the term dispersion will be understood to designate an intimate suspension of finely-divided solid particles in a liquid, whereas the term emulsion will be applied only to intimate suspensions of finely-divided liquid droplets in another liquid.

A fiber having the desirable properties of polytetrafluoroethylene, e.g., chemical inertness, high temperature stability, non-adhesiveness, low flexibility, low wettability with water and organic liquid, low coefficient of friction and unique electrical properties, has promising utility for many industrial purposes. However, the chemical inertness of polytetrafluoroethylene (apparent from its low solubility) and its high temperature stability (apparent from its high melting point) have made the processing of this polymer difficult. It is therefore obvious that polytetrafiuoroethylene cannot be spun into fibers by conventional spinning methods, e.g., by wet or dry-spinning from solution or by melt extrusion. A newer method involving the spinning of polytetrafiuoroethylene in the form of a lubricated paste, requires enormous pressures, and yields only coarse fibers of uneven denier and extremely low strength before sintering. These high pressures, together with the unadaptability of the polymer to the spinning of multi-filament yarns composed of low denier fibers, and the necessity of using the batch process in the application of the lubricated paste process impose serious limitations on the use of this process. Other known techniques for forming continuous filaments of polytetrafiuoroethylene have resulted in very weak filaments of no practical value; the filaments were not well formed and their weakness before they were to be subjected to the sintering applied to this type of fiber prevented handling by conventional methods. An approach to the problem of the dry spinning of polytetrafiuoroethylene dispersions by transferring the aqueous polytetrafiuoroethylene dispersion into an organic liquid has hitherto been unsuccessful; this attempt resulted in the formation of a coagulated non-filterahle paste which could not be diluted with additional dispersant.

It is, accordingly, an object of the present invention to provide a process for making shaped articles from polytetrafiuoroethylene. A further object is to provide new compositions of matter for use in the preparation of shaped articles, particularly fibers and film, of polytetrafluoroethylene. A still further object is to provide a process for making strong textile filaments from polytetrafluoroethylene. Other objects will appear hereinafter.

These and other objects are accomplished by emulsifying a solution of a matrix polymer in an organic, waterimmiscible solvent, into an aqueous dispersion of polytetrafiuoroethylene. Such a mixture can properly be considered an oil-in-Water emulsion and contains substantially the following four components: water, a water immiscible organic solvent, polytetrafluoroethylene, and a matrix polymer.

This emulsion comprises an aqueous colloidally dispersed spinnable polytetrafluoroethylene composition in contrast to compositions which are water-free but are invariably coagulated. The combined water-organic solvent system has permitted the use of polymers as a matrix for the polytetrafiuoroethylene not possible in a dispersion of polytetrafluoroethylene alone. In particular, this emulsion makes possible the use of polytetrafiuoroethylene in dry spinning and in wet spinning processes.

The production of film or fibers from this emulsion/ dispersion of polytetrafiuoroethylene, in accordance with the present invention, involves the following steps: extrusion of the emulsion through a conventional extrusion orifice or orifices, e.g., an elongated extrusion slot or a spinneret, into a setting medium for the matrix material, sintering of the contained polytetrafiuoroethylene at a temperature about its crystalline melt temperature (327 C.), and elimination of the matrix material if necessary or desired. The last-named step, in most cases, will not have to be carried out as a separate step, depending on the selection of matrix material, since at the sintering temperature of polytetrafinoroethylene, most commonly used matrices decompose into volatile monomeric materials or flash off as low molecular polymers.

The strength of the fibers or film during spinning is due, in large part, to the matrix polymer which coagulates and coalesces after being extruded through the spinneret, thereby enveloping therein discrete particles of the insoluble, unmelted polytetrafluoroethylene.

As a setting medium, air or methanol has been found satisfactory for all of the selected matrices, although equivalent gaseous and liquid coagulants may be used.

For the purpose of making the emulsion/dispersion smooth and stable, 0.05 to 5% inclusive of a surfactant, i.e, surface active agent, by weight of the combined weights of liquids, is added to the above four component system. It has been found that any surfactant yielding an oil-in-water emulsion can be used. Examples of some surfactants which can be used are listed in Hochberg U.S. Patent No. 2,681,324, issued June 15, 1954.

The invention will be more clearly understood by reference to the examples and the discussion which follow. These examples are given for illustration only and are not limitative; parts, proportions and percentages are by weight unless otherwise indicated.

Example I Three grams of an alkyl aryl polyether alcohol sold under the trade name Triton X- and 10 grams polystyrene were dissolved in 37 grams of toluene and added to 50 grams of a stirred aqueous dispersion of 60% polytetrafiuoroethylene. The stirring was continued until emulsification took place. Two grams methyl cellulose with stirring were added to form a smooth cream. This composition was dry spun, i.e., spun by the evaporative system by which the polymer is coagulated by evaporation of the liquid of the spinning composition, through a spinneret having 3 holes of 0.2 mm. diameter each into a one-foot muffie furnace at C. to form self-supporting, continuous fibers which during their spinning may be free of support, during their travel for four feet of their length. The yarn was wound up and sintered to a brown yarn product having about 2 drawability, i.e., drawable to 3 times its undrawn length, at room temperature. A similar composition was wet-spun into Warm methanol to form fibers of moderate strength before sintering.

Example ll 50 cc. of an aqueous polytetrafiuoroethylene dispersion containing 60% polytetrafluoroethylene and 6% 3 Triton X-100 polyether alcohol were mixed with 50 cc. of a 10% polystyrene solution in carbon tetrachloride. Ten grams Triton X-100 polyether alcohol were added, but this mixture had too low a viscosity for proper spinning. After the addition of 3 grams sodium pectinate, the mixture was readily spinnable into continuous yarns.

Example Ill 50 cc. of a 60% aqueous polytetrafiuoroethylene dispersion containing 6% Triton X-100 polyether alcohol were mixed with 50 cc. of a 10% polystyrene solution in toluene. The mixture was stirred with sodium lauryl sulfate and the viscosity increased by adding three grams methyl cellulose.

When spun into air, continuous filaments could be obtained which may be free of support, during their travel, for three feet of the thread length. These filaments were wet and soft and could not be wound up. By spinning it into methanol of 50 C., the filaments could be easily wound up on a bobbin.

Example IV An oil-in-water type emulsion was formed by mixing 100 grams of a 15% polyisobutylene solution in toluene, 3 grams polyoxyethylene sorbitan monolaurate, and 100 grams of an aqueous 60% polytetrafluoroethylene dispersion. The smooth uniform emulsion was dilutable with water, and was spun through a 5-hole spinneret, having hole diameters of 0.2 mm. at the speed of 20 yards per minute through a two-foot bath of warm (55 C.) methanol. The yarn was wound up on a bobbin and, after sintering by passing it successively over and in contact with two electrically heated rollers for a contacting period of to seconds at 380 C., a strong, white polytetrafluoroethylene yarn formed, substantially free of matrix polymer.

Example V One hundred grams of a 10% poly(vinyl toluene) solution in toluene was mixed with 4 grams of glycerol oleate and 100 grams of a 60% aqueous polytetrafluoroethylene dispersion. The mixture was stirred to form a smooth emulsion. This oil-in-water emulsion was extruded, after increasing the viscosity by adding 3 grams of sodium alginate, through a 3-hole spinneret (hole diameter 0.2 mm.) into methanol of 55 C. The spinning strength was contributed by the poly(vinyl toluene) matrix, which, after sintering of the polytetrafluoroethylene component successively over and in contact with electrically heated rolls at 360 C. flashes off, leaving a strong, almost white polytetrafluoroethylene filament. This filament was drawn over and in contact with a hot plate at 425 C. to about 225% (TAX) of its as-spun length to give a filament of a tenacity of 0.5 gram per denier.

As can be seen from the above examples, complete coalescence of the polymer particles is achieved by sintering. Development of optimum mechanical properties is dependent in part upon the sintering condition, since incomplete sintering results in weak spots with attendant poor mechanical properties. The optimum sintering temperature for the developing of maximum properties appears to be approximately 350 to 400 C. At this temperature, yarns have to be sintered about 7 seconds before maximum physical properties can be developed. While higher sintering temperatures naturally require shorter sintering times, and sintering temperatures up to 430 C. have been used successfully, the contact times at the temperatures below about 375 C. required to develop maximum properties become excessive. Many suitable heating media, such as a molten salt or metal baths, heated rolls or plates, hot air, or radiant heat may be used for the sintering step.

Suitable tensile properties for commercial applications are obtained by drawing the filaments after sintering,

preferably at temperatures between the crystalline melting point (327 C.) and the decomposition temperature of the tetrafluoroethylene polymer. For sintering temperatures, 430 C. represent a practical upper limit, since tetrafluoroethylene polymer degradation begins to become appreciable at this temperature. When sintering and drawing are combined into a single operation, temperatures of approximately 400 C. represent about the best balance between sintering rate, drawability, decomposition, and the yarn properties. Where drawing is performed as an operation separate from sintering, it is preferably carried out at temperatures between 330 C. and 400 C.

The invention has particularly been described in terms of tetrafluoroethylene homopolymer, also called polytetrafluoroethylene made in the absence of materials copolymerizable with tetrafluoroethylene, and this is the preferred polymer for use in the practice of the invention, and it is the preferred polymer for use in this process. However, the invention also applies to other polyhalogenated polymers. For example, copolymers of tetrafluoroethylene with other unsaturated organic compounds, such as ethylene, perfiuoropropylene and chlorotrifluoroethylene may be used. The copolymerizable modifier may be present in the copolymer up to about 15% by weight of the copolymer and still maintain the essential properties of the tetrafluoroethylene homopolymer. Telomerized, i.e., long chain, tetrafluoroethylene polymers in which the end groups of the polymer chain are supplied by non-polymerizable compounds, e.g., methanol and ethanol, present during the polymerization to terminate the polymerization at the desired molecular weight may also be used.

The tertrafluoroethylene polymer can vary widely as to molecular weight. Generally speaking, the preferred molecular weights for the tetrafluoroethylene polymer are those of 8000 and higher.

While the production of the tetrafluoroethylene polymer dispersions is not a part of the present invention, they may be prepared to any suitable process described in the prior art, for example, according to the procedures of Llewellyn and Lontz U.S. Patent Numbers 2,685,707, issued August 10, 1954, Berry U.S. Patent No. 2,559,750 issued July 10, 1951, Renfrew U.S. Patent 2,534,058 issued December 12, 1950, or Berry U.S. Patent 2,478,229 issued August 9, 1949.

While the particle size of the tetrafluoroethylene polymer in a dispersion may vary over a wide range, it is perferable that the particles of polymer be colloidal and sufficiently small to pass through the holes of a spinneret; the particle size range of 0.05 to 5 microns and preferably 0.1 to 2 microns are suitable for the practice of the invention. The smaller the particles, the more easily the mixture can be extruded.

A great deal of water in the emulsion complicates the spinning process. Two methods have been found to avoid this complication. The viscosity cound be increased (a) by addition of small amounts of a water-soluble polymer, or (b) by evaporation of part of the water. Both these methods produce an emulsion spinnable by wet and dry spinning processes. The first method, however, is preferred, since the water-soluble polymer facilitates the formation of a threadline from the matrix polymer immediately upon extrusion. It has been demonstrated that this water-soluble polymer is insufficient to form a threadline by itself, in the amounts used and therefore cannot be used as a matrix material alone. As water-soluble poly mers to be added as thickeners, methyl cellulose, polyvinyl alcohol, sodium alginate, sodium pectinate, and the like, are suggested. The other method, consisting of evaporating some of the water from the emulsion dispersion, is more complicated in its application, since, after reaching a minimum amount of water, the emulsion tends to invert or becomes thick, stringy, and unspinnable. A total solids content of over 30% of the combined weight of the dispersion components has been found satisfactory. The preferred solids content, however, is above about 38%, since dispersions of such concentrations can easily be dry spun, while dispersions containing from about 30 to 38% solids have to be wet spun. In wet-spinning a lower percentage of solids is required, as the setting medium chosen is a solvent for the organic, water-immiscible solvent, thus concentrating the solids of the extruded mixture with a solidification of the matrix polymer containing embodied therein discrete particles of the intractable polytetrafiuoroethylene polymer.

Another important condition concerns the composition of the solids in the emulsion/dispersion. The ratio of polytetrafluoroethylene/matrix polymer may range from 2:1 to :1, with the preferred range being from 3:1 to 8:1. Too low a polytetrafluoroethylene content yields poor fibers after sintering. At lower ratios the emulsion tends to invert or becomes unspinnable. A high ratio of polytetrafluoroethylene dispersion to matrix solution can readily be attained by the addition of a surfactant.

Any surface-active agent which is known to promote the emulsification of oil in water may be used in the practice of this invention. A great many surfactants, sold as emulsifiers, have been tried and found very satisfactory in the practice of the present invention; among these are polyethylene glycol esters of fatty alcohols, diaryl sulfonates, aryl alkyl sulfonates, alkyl aryl polyether alcohols, polyglycol esters, fatty alkylol amine condensates, aliphatic ester sulfates, glycerol monoesters from fatty acids, polyoxyethylene alkyl ethers, soya lecithin, phosphates of fatty alcohols, tertiary amine salts, etc. An addition of 1 to 4% (by weight of the polymer solution) of such a surfactant allows polytetrafluoroethylene dispersion/ 10% matrix polymer solution ratios up on 1.5 :1 which, in other words, means a ratio of polytetrafluoroethylene matrix polymer up to about 9: 1. The most useful ranges for a good emulsion and for satisfactory spinning appeared to be 3:1 to 8:1. Unstabilized 30% polytetrafluoroethylene dispersion was also useable for preparing emulsions, in which case less surfactant sufliced. Use of this dispersion, however, introduced much more water into the emulsion and appeared to be unattractive.

Matrix materials useful in practicing the present invention are all those polymers which are soluble in an organic, water-immiscible solvent. The preferred matrix materials for practicing the present invention have a decomposition temperature below 300 C. and flash off at such a temperature without leaving charred remains. Examples of useful matrix materials are polystyrene, polyisobutylene, polyvinyl toluene, nitrocellulose, polyethylene, or copolymers like a butadiene/ styrene copolymer. Some of these matrix materials are even traded in their solution form in an organic water-immiscible solvent. These matrix materials are generally characterized by their ability to deposit a coherenet, homogeneous film from solution after removal of the solvent.

Among the organic solvents useful in practicing this invention are heptane, toluene, carbon tetrachloride, chloroform, benzene, liquid aliphatic hydrocarbons or their halogen derivatives, water-immiscible ethers, waterimmiscible esters or ketones, nitrobenzene, chlorobenzene, xylenes, etc. These solvents are substantially immiscible in water since their solubility in water is small and the bulk of this solvent is undissolved in the water of the emulsion. Generally speaking, however, any liquid which will dissolve the matrix polymer is satisfactory for the practice of the invention, provided the solvent is substantially immiscible with water.

The new compositions of matter of the present invention are clearly distinguished from the formerly known dispersion spinning mixtures. In the case of these new compositions, water-insoluble organic polymers can be used as matrix materials. Many of these matrix materials, in contrast to water soluble polymers, revert to volatile materials at sintering temperatures of polytetrafluoroethylene; such matrix material has the valuable and desired characteristic of flashing off during sintering without leaving any charred remains, thus yielding a white polytetrafluoroethylene structure which cannot be obtained by using polymeric water-soluble matrices. The continuous phase in these compositions is the water, which means that the emulsions are dilutable with water to any desired extent. These compositions can be kept for months and no agglomeration occurs; they are fiber forming with about the same ease as a cellulose acetate spin dope, and they are readily dry-spun.

Any departure from the above description which conforms to the spirit of the present invention is also intended to be included within the scope of the claims.

I claim:

1. The process which comprises commingling an aqueous dispersion of polytetrafluoroethylene particles of a size within the range of from about 0.05 to about 5 microns with a solution of water-insoluble film-forming polymer selected from the group consisting of polystyrene, polyisobutylene, polyvinyl toluene, polyethylene, and butadiene-styrene copolymers, in a water-immiscible organic solvent selected from the group consisting of heptane, toluene, carbon tetrachloride, chloroform, benzene, nitrobenzene, and chlorobenzene; thoroughly agitating the resulting mixture in the presence of a surface-active agent which promotes the emulsifying of oil-in-water selected from the group consisting of polyethylene glycol esters of fatty alcohols, diaryl sulfonates, aryl alkyl sulfonates, alkylaryl polyether alcohols, polyglycol esters, fatty alkylol amine condensates, aliphatic ester sulfates, glycerol monoesters from fatty acids, polyoxyethylene alkyl ethers, soya lecithin, phosphates of fatty alcohols, and tertiary amine salts to provide an emulsion of the oil-in-water type wherein the ratio of polytetrafluoroethylene to film-forming polymer is from about 2:1 to about 10:1; incorporating a water-soluble thickener polymer selected from the group consisting of methyl cellulose, polyvinyl alcohol, sodium alginate and sodium pectorate; and spinning the resulting emulsion into a filament.

2. The process of claim 1 wherein the said filament is thereafter heated to a temperature from about 350 C. to about 430 C. for a period of at least about 7 seconds.

References Cited in the file of this patent UNITED STATES PATENTS 2,530,362 Morris Nov. 14, 1950 2,671,065 Ulrich Mar. 2, 1954 2,681,324 Hoclrberg June 15, 1954 2,718,452 Lontz Sept. 20, 1955 2,737,436 Le Boeuf Mar. 6, 1956 2,766,214 Erchak et al Oct. 9, 1956 2,806,256 Smith-Johannsen Sept. 17, 1957 2,868,741 Chamber et 'al Jan. 13, 1959 FOREIGN PATENTS 588,697 Great Britain May 30, 1947 486,628 Canada Sept. 16, 1952 511,376 Canada Mar. 29, 1955 

1. THE PROCESS WHICH COMPRISES COMMINGLING AN AQUEOUS DISPERSION OF POLYTETRAFLUORETHYLENE PARTICLES OF A SIZE WITHIN THE RANGE OF FROM ABOUT 0.05 TO ABOUT 5 MICRONS WITH A SOLUTION OF WATER-INSOLUBLE FILM-FORMING POLYMER SELECTED FROM THE GROUP CONSISTING OF POLYSTYRENE, POLYISOBUTYLENE, POLYVINYL TOLUENE, POLYETHYLENE, AND BUTADIENE-STYRENE COPOLYMERS, IN A WATER-IMMISCIBLE ORGANIC SOLVENT SELECTED FROM THE GROUP CONSISTING OF HEPTANE, TOLUENE, CARBON TETRACHLORIDE, CHLOROFORM, BENZENE, NITROBENZENE, AND CHLOROBENZENE; THROUGHLY AGITATING THE RESUTLING MIXTURE IN THE PRESENCE OF A SURFACE-ACTIVE AGENT WHICH PROMOTES THE EMULSIFYING OF OIL-IN-WATER SELECTED FROM THE GROUP CONSISTING OF POLYETHYLENE GLYCOL ESTERS OF FATTY ALCOHOLS, DIARYL SULFONATES, ARYL ALKYL SULFONATES, ALKYLARYL POLYETHER ALCOHOLS, POLYGLYCOL ESTERS, FATTY ALKYLOL AMINE CONDENSATES, ALIPHATIC ESTER SULFATES, GLYCEROL MONOESTERS FROM FATTY ACIDS, POLYOXYETHYLENE ALKYL ETHERS, SOYA LECITHIN, PHOSPHATES OF FATTY ALCOHOLS, AND TERTIARY AMINE SALTS TO PROVIDE AN EMULSION OF THE OIL-IN-WATER TYPE WHEREIN THE RATION OF POLYETTRAFLUOROETHYLENE TO FILM-FORMING POLYMER IS FROM ABOUT 2:1 TO ABOUT 10:1; INCORPORATING A WATER-SOLUBLE THICKENER POLYMER SELECTED FROM THE GROUP CONSISTING OF METHYL CELLULOSE, POLYVINYL ALCOHOL, SODIUM ALGINATE AND SODIUM PECTORATE; AND SPINNING THE RESULTING EMULSION INTO A FILAMENT. 